This is currently proxied and responding with 400 Bad Request to every request, albeit with working SSL.
If you were using DNS-only mode, then you’d just need to configure a valid SSL Certificate at your origin. Cloudflare doesn’t get in the middle of your connection in DNS-only mode at all, it’s just up to your origin to respond right.
You’ll have to wait for DNS Cache to expire after changing between Proxied and DNS-only as well, which can take as little as a few minutes and as high as an hour or so depending on your DNS Resolver (some ISP operated DNS Resolvers, which are often defaults, apply their own cache times/TTLs). You can look at the chrome/firefox dev tools to see if it proxied or not, both will show you the IP Address you are connecting to for each request.